Continuous monitoring of haemodynamic parameters in humans during the early phase of simulated diving with and without breathholding

This study examined the integrative changes of blood pressure (BP) and stroke volume (SV) leading to the initial biphasic heart rate (f(c)) response (first 15 s) in simulated diving manoeuvres with and without breath-holding (BH). Simulated diving was studied in ten young healthy volunteers by application of a gel-filled pack at 0°C and 18°C on the forehead with and without BH. Beat-by-beat and second-to-second f(c), BP, SV, and total peripheral vascular resistance (TPR) were followed by continuous non-invasive monitoring. In all conditions (BH with forehead cooling at 0°and 18°C) there was an early rise in BP triggering the first tachycardial response (f(c) acceleration) which was immediately counteracted by the concurrent further increase of SV leading to the second phase of early bradycardic response (f(c) deceleration). Furthermore, the continuous beat-by-beat and second-to-second monitoring allowed the documentation of a highly significant increase of TPR within the first few seconds of the manoeuvres. Our data further indicated that the differences in haemodynamics observed during the stimuli at different temperatures was overruled by BH. Detailed comparisons of the beat-by-beat and second-to-second analyses were unable to show that one method was better than the other. Using continuous non-invasive monitoring of haemodynamic variables during simulated diving manoeuvres it was possible to provide better insights into the physiological principles and meaning of the diving reflex in humans.